Preheat oven to 350 degrees F (175 degrees C). Grease two 12 cup muffin pans or line with paper baking cups.
In a medium bowl, stir together the rice flour, millet flour, cocoa, salt, baking soda, baking powder and xanathan gum. In a separate large bowl, beat the eggs, sugar, sour cream, milk and vanilla. Stir in the dry ingredients until smooth. Spoon the batter into the prepared cups, dividing evenly.
Bake in the preheated oven until the tops spring back when lightly pressed, 20 to 25 minutes. Cool in the pan set over a wire rack. When cool, arrange the cupcakes on a serving platter.